(1) Field of the Invention
The present invention relates to a backlight unit and a LCD (Liquid Crystal Display) apparatus, especially to a backlight unit which is disposed at the back of a LCD panel to constitute a LCD apparatus, and to the LCD apparatus including the backlight unit.
(2) Description of the Related Art
In recent years, as the LCD apparatuses have started to be employed in liquid crystal televisions or the like in full scale, the demand for the backlight units, which are mounted in the LCD apparatuses, has increased as well.
Backlight units are classified into two types: an edge-light type (also referred to as a side-light type or an optical waveguide type) in which an optical waveguide is disposed on the back surface of a LCD panel, and fluorescent lamps are arranged at the edges of the optical waveguide; and a direct-below type in which a plurality of fluorescent lamps are arranged on the back surface of a LCD panel to be in parallel with the back surface. In general, edge-light type backlight units are advantageous in achieving a thin body and even brightness on the light-emitting surface, but are disadvantageous in achieving high brightness, and the direct-below-type backlight units are advantageous in achieving high brightness, but are disadvantageous in achieving a thin body.
Accordingly, LCD apparatuses for liquid crystal televisions, which put weight on achieving high brightness, often adopt the direct-below-type backlight unit.
A direct-below-type backlight unit has a reflection plate, which is in a rectangular shape that corresponds to a horizontally wide screen of the liquid crystal television, and a translucent plate that includes an optical diffusion plate, the plates being arranged in parallel with each other. The direct-below-type backlight unit also has a plurality of fluorescent lamps that are arranged between the reflection plate and the translucent plate, and lets light, which is emitted from the fluorescent lamps, pass through the translucent plate toward the LCD panel. With such a construction, the LCD panel receives light at its back from the backlight unit. The four sides of the backlight unit between the reflection plate and the translucent plate, in which the fluorescent lamps are arranged, are closed by side plates or the like to prevent dusts or the like from entering into the backlight unit (see, for example, Japanese Laid-Open Patent Application No. 2002-214605).
In regards with the backlight unit having the above-described construction, many types of backlight units, which are different in the shape or layout of the fluorescent lamps, have been put into practical use. Such backlight units include: a backlight unit in which straight-tube fluorescent lamps are laid horizontally with regular intervals in the vertical direction (hereinafter, the backlight unit is referred to as a straight-tube transverse-mounted type); a backlight unit in which straight-tube fluorescent lamps are erected vertically with regular intervals in the horizontal direction (hereinafter, the backlight unit is referred to as a straight-tube vertical-mounted type—see the above-mentioned Japanese Laid-Open Patent Application No. 2002-214605); and a backlight unit in which fluorescent lamps curved into the shape of character “U” are laid horizontally with regular intervals in the vertical direction (hereinafter, the backlight unit is referred to as a curved-tube transverse-mounted type—see Japanese Laid-Open Patent Application No. 7-270786).
Meanwhile, as the liquid crystal television are becoming larger in size and higher in brightness, the number of fluorescent lamps per direct-below type backlight unit to be attached to a LCD panel for a liquid crystal television is increasing. As the number of fluorescent lamps increases, the temperature in the backlight unit increases. This causes the temperature distribution in the backlight unit to be more uneven. In general, the liquid crystal televisions with horizontally wide screens are erected vertically in use. As a result, the upper portion of the backlight unit has a higher temperature, and the lower portion has a lower temperature.
In the transverse-mounted type backlight units in the above-described cases, the brightness is higher at a fluorescent lamp arranged at a higher position, and is lower at a fluorescent lamp arranged at a lower position. This makes a difference in brightness between fluorescent lamps and causes uneven brightness in the overall backlight unit. In the case of the straight-tube vertical-mounted type, there is hardly a difference in brightness between the lamps, but the brightness is higher at higher portions and is lower at lower portions of each lamp. This causes uneven brightness in the overall backlight unit.